The rubbers mentioned above are very commonly used now in the rubber industry, and these rubbers are all cured with sulfur as a main curing agent. Where it is desired to improve the thermal resistance and aging resistance of vulcanized rubbers, the use of a sulfur donor as a curing agent is recommended. In many cases, the sulfur donor is used in combination with sulfur.
It has previously been believed that in sulfur vulcanization, the use of curing accelerators and activators together with sulfur is essential. The vulcanization accelerator is a substance which when used in small amounts, can increase the rate of vulcanization and decrease the temperature and time necessary for vulcanization. The accelerator produces other beneficial results such as the improvement of the physical properties of vulcanized rubbers, such as tensile strength or modulus, the preventation of blooming, or the improvement of their aging resistance. The activators, on the other hand, are substances which when used together with vulcanization accelerators, enable the vulcanization accelerators to exhibit their activities effectively, and affect the rate of vulcanization and the physical properties and aging characteristics of rubber. Vulcanization with sulfur alone without accelerators is scarcely acceptable for commercial operation because the rate of vulcanization is slow, and the resulting vulcanized rubbers have poor physical properties. Conventional accelerators, however, must be used together with activators in order to exhibit satisfactory accelerating effects, and in the absence of activators, the results are extremely unsatisfactory. For this reason, it has been believed in the rubber industry that sulfur, accelerators and activators are the three essential elements for vulcanizing rubbers.
Typical examples of the vulcanization accelerators which have been used in the art are guanidines such as diphenyl guanidine, aldehyde-amines such as a condensation product of n-butyraldehyde and butylidene aniline, aldehyde-ammonias such as hexamethylene tetramine and acetaldehyde ammonia, thiazoles such as 2-mercapto-benzothiazole, sulfenamides such as benzothiazyl-2-diethyl sulfenamide and N-cyclohexyl-2-benzothiazyl sulfenamide, thiurams such as tetramethylthiuram disulfide and dipentamethylenethiuram hexasulfide, dithiocarbamates such as sodium dimethyl dithiocarbamate and zinc dimethyl dithiocarbamate, and xanthates such as sodium isopropyl xanthate. All of these accelerators must be used in conjunction with activators (mainly zinc oxide). Some of these conventional accelerators have known or suspected toxicity, and from the standpoint of controlling environmental pollution, the development of accelerators not injurious to man's health and having a high degree of safety has been desired.
Among the conventional activators are zinc oxide, litharge, magnesium oxide, organic amines, alkali carbonates, and alkali hydroxides. Of these, zinc oxide is the best activator and gains almost exclusive acceptance in the rubber industry. The zinc oxide, however, is liable to scatter as dust, and its use is undesirable for the health of the working personnel. Zinc oxide added as an activator also remains in the final vulcanized rubber product, and dissolves out in service. Accordingly, the use of zinc oxide is especially undesirable where the vulcanized products are used in applications associated with medical therapy and foodstuffs (for example, as bottle stoppers, nipples or nursing bottles, or cap liners). It is very desirable therefore to develop accelerators which do not require the conjoint use of activators typified by zinc oxide.
It is well known on the other hand that when the sulfur donor is used as a curing agent either alone or in combination with sulfur, the conjoint use of zinc oxide is essential. Accordingly, the same hygienic problem exists in this case.
It has now been found that specified amino acids exhibit very good vulcanization accelerating effects when vulcanizing the aforementioned rubbers with sulfur and/or a sulfur donor, and surprisingly, the use of the amino acids quite obviates the conjoint use of activators which is essential in the case of conventional vulcanization accelerators.